Generally, batteries can be divided into chemical batteries and physical batteries, and chemical batteries can further be divided into primary batteries, secondary batteries and fuel batteries.
Secondary batteries, also known as rechargeable batteries, are charged/recharged by applying an electric current, which reverses the chemical reactions that occur during discharge/use. Common secondary batteries include nickel metal hydride battery (NiMH battery or nickel hydride battery), lead-acid battery and lithium ion battery. The lead-acid battery, though the oldest form of secondary batteries, still remains popular for its good reliability, low cost for manufacture and purchase, and high regeneration rate.
A conventional lead-acid battery comprises a negative electrode of metallic lead, a positive electrode of lead dioxide, and a diluent sulfuric acid electrolyte. The chemical reactions during battery discharge are indicated below:
Reaction at the negative electrode:Pb(s)+SO42−(aq)→PbSO4(s)+2e−
Reaction at the positive electrode:PbO2(s)+SO42−(aq)+4H++2e−→PbSO4(s)+2H2O
As the lead-acid battery discharges, electrons are released from the negative electrode and the resulting lead ions (Pb2+) immediately react with sulfate ions (SO42−) and form insoluble lead sulfate (PbSO4) crystals adsorbed on the surface of the negative electrode. At the positive electrode, electrons from the external circuit reduce PbO2 to Pb2+, which also react with SO42− and form PbSO4 crystals adsorbed on the positive electrode. By applying an opposite voltage, reverse chemical reactions occur and then the battery recharges.
In practice, the lead sulfate formed on the electrodes cannot be completely converted to lead ions and sulfate ions during the recharge cycles, and therefore the amount of the sulfate ions in the electrolyte will gradually decrease. This problem would become more severe when a coarse lead sulfate cluster forms due to deep discharge or rapid recharge of the battery. The residual lead sulfate degrades the cooling rate of the electrodes and reduces the effective surface area of the Pb and PbO2 electrodes, thereby reducing the capacity and life cycle of the battery. Another problem is water loss due to gas evolution, which happens when the water contained in an electrolyte solution is electrolyzed during deep discharge or rapid recharge/charge of the battery or is evaporated due to heat accumulated in the battery. The water loss makes it harder to dissolve lead sulfate; the oxygen and hydrogen produced during the electrolysis of water jeopardizes the safety of the battery. Furthermore, the presence of sulfuric acid concentration gradient increases the internal resistance of the battery, decreases the mobility of the ions, and thus adversely affects the performance of the battery.
Therefore, improvements addressing the above mentioned problems have been sought for a long time.
In order to solve the above problems, various battery additives have been employed to modify the electrodes, especially additives of carbon material, such as carbon black, activated carbon, carbon nanotubes and graphene. For example, CN 102201575 discloses a lead sulfate-graphene composite electrode material and a negative paste comprising the same; CN 101719563 discloses a lead-acid battery with graphene added to the negative electrode; US 20120328940 A1 discloses the use of carbon nanotubes or graphene as an additive in the electrodes; CN 1505186 and US 2005181282 disclose use of carbon nanotubes for the cathode and anode of lead-acid batteries. It is believed that using carbon nanotubes and/or graphene in the electrodes can improve the properties of the lead-acid battery.
Much effort has been directed to the use of the above-mentioned carbon material, especially carbon nanotubes and/or graphene, in battery electrodes, but it still cannot effectively improve the problem of polarization caused by the sulfuric acid concentration gradient. Moreover, in the conventional processes for preparing modified electrodes with carbon material, a higher amount of carbon material (about 5 wt % based on the total weight of the lead paste) is required and the processes generally involve complicated steps and therefore involve higher cost. It is thus still necessary to develop a cheaper and simpler approach to provide a battery with improved performance.